1. Field of the invention
The present invention relates to injection molding of three-layered containers for dry foods and other food such as instant coffee and powdered cream which require vapor-proof containers of very low gas permeability, a method, and an apparatus thereof by means of injection molding.
2. Prior Art
Although plastic containers are widely used for food, medicine, cosmetics, etc., it has been necessary to improve the gas-barrier properties of plastic containers when they are used as containers for dry foods and other foods, such as instant coffee and powdered cream, which tend to deteriorate in the presence of oxygen and water vapor.
Highly protective multilayered containers prepared by laminating gas barrier resin layers such as ethylene-vinyl alcohol copolymers can be cited as plastic containers having highly protective gas-barrier properties. A multilayer injection molding apparatus disclosed in Japanese Patent Application First Publication "kokai" No. 59-67029 can be cited as an example of an apparatus for molding parisons for such multilayered containers, and injection molding of multilayered bottomed parisons disclosed in Japanese Patent Application First Publication "kokai" No. 61-37404 can be mentioned as an example of a method for molding parisons described above.
In the multilayer injection molding apparatus described above, a nozzle is inserted inside an outer nozzle to give a resin path both inside and outside the inserted nozzle, and each of the resins introduced through these paths are injected from the gate of the outer nozzle.
Such an injection molding apparatus has the general structure shown in FIG. 7. An outer injection nozzle 4 of an outer injector element 3, which injects resin into a metal mold 1 of the machine, is connected to an orifice 2 formed at the bottom of the metal mold 1; and an inner injector element 5 is inserted in the outer injector element 3 to form resin flow paths 6 and 7 on the outside and inside, respectively, of the inner injector element 5. For example, polyethylene terephthalate type resin and ethylene vinyl alcohol copolymer are injected into the metal mold 1 from the outer injection nozzle 4 by simultaneously introducing the former resin into the outer injection nozzle 4 through the resin flow path 6 and the latter resin into the outer injection nozzle 4 through the resin flow path 7.
The injection molding described above of the multilayered parisons can be carried out to produce 5- or 4-layered bottomed parisons by using a triple nozzle having three concentric flow paths inside the nozzle body mouth, forming an inner and an outer layer with resins flowing out from the outer flow paths, and forming middle layers with resins flowing out from the two internal flow paths.
In the conventional injection molding described above, two or more kinds of resin are simultaneously injected simply by placing another resin flow path on the outside of an inner resin flow path. The obtained parisons show, therefore, uneven thickness of each layer and varied height of the middle layers in the circumferential direction, thus the formation of necessary middle layers is difficult. Multilayered containers with high gas-barrier properties cannot be mass-produced due to the impossibility of continually forming container walls with high gas-barrier properties when such an injection molding method is employed, and the multilayered containers so produced do not have an attractive appearance.
The resin for the outer wall of the containers is usually supplied from the side of the nozzle body, and the resin feed to the side opposite to the resin supply-side becomes insufficient due to the flow path length difference between the resin supply-side flow and the opposite-side flow. In addition to this, injection conditions cannot be held constant due to indefinite setting of fluidity of the resin forming each layer, and therefore manufacture of many products having a uniformly high quality has been extremely difficult.
Furthermore, the ethylene-vinyl alcohol copolymer employed for the middle layer of the multilayered containers tends to be thermally decomposed easily, and therefore may suffer thermal damage in the resin flow path during injection molding. Moreover, the flow temperature of ethylene-vinyl alcohol copolymer must be 50.degree.-60.degree. C. lower than that of the polyethylene terephthalate type resin used for the inner and outer layers. Ethylene-vinyl alcohol copolymer may deteriorate from heat conducted from the polyethylene terephthalate when many parisons are simultaneously produced by injection in a molding apparatus having branched and closely spaced resin supply paths. The continuous mass production of the multilayered containers having desired properties and quality therefore becomes difficult, and considerable variation in quality is observed among simultaneously injected parisons. The quality of parisons also shows wide variation among sets.